1. Field of the Invention
The present invention relates to a mobile communication system, and in particular to a mobile station that obtains location information by communicating with a base station and a switch, and a base station that calculates distance from a mobile station to which the asynchronous CDMA system is applied.
2. Description of Related Art
Generally, a mobile communication system, represented by a portable telephone system, comprises a mobile station for user to communicate while moving within a radio area or moving from one radio area to another, a base station that forms a radio area by providing a certain area with radio resources for communication, and a switch that forms a communication network by controlling a plurality of base stations and mobile stations in the radio areas of those base stations and connects with other networks including a fixed network. In such a mobile communication system, the base station control function of the switch may be separated from it as a base station control apparatus, which is installed between the base station and the switch. In the following, however, for the sake of convenience of explanation, simply the switch is referred to.
In a mobile communication system, there is a demand for specifying a location of a sender in an emergency or when a mobile station is used illegally. Further, also in ordinary use, it is advantageous to specify a users detailed location, since service such as guidance information can be offered in accordance with that location. Thus, it is very useful to be able to obtain location information of a mobile station.
As a method of obtaining location information of a mobile station, there are mentioned, for example, a method of utilizing a marker embedded in a road or building, and a method of using GPS (Global Positioning System), which is a position system using satellites and used for a car navigation system etc.
However, in the mentioned systems, special devices are required, and communication for a mobile station to obtain location information should be made differently from the ordinary communication, making the processing load relatively large.
As conventional technique that solve the above-mentioned drawbacks for obtaining location information of a mobile station, Japanese Unexamined Patent Application Laid-Open Nos. 7-181242 and 10-322752 are known, for example. These conventional techniques are ones in which location information of a mobile station is obtained by using information on the radio area to which the mobile station belongs and a propagation state of an ordinary channel for mobile communication. These techniques are methods that utilize high-precision timing information in a mobile communication system using a communication channel having that high-precision timing information.
The conventional technique described in the former patent application document 7-181242 is a method in which base stations transmit channels of the same type such that transmission timing differences among base stations are values known in advance, and a mobile station obtains a propagation delay difference for each of plurality of channels by subtracting a transmission timing difference from a reception timing difference, in order to obtain respective differences in distances between the mobile station and the base stations. Further, the conventional technique described in the latter patent application document 10-322752 is a method in which forward-and-return transmission/reception are made between a mobile station and a base station, and the time needed for this transmission-and-reception is measured to obtain a propagation delay and thus to obtain a distance between the mobile station and the base station.
The conventional techniques described in the above two patent application documents premise that a communication channel having high precision timing information is used. As such a representative mobile communication system that uses a communication channel having high precision timing information, a mobile communication system using CDMA (Code Division Multiple Access) is known. CDMA is a technique in which a communication channel is coded with a signal sequence, called a diffusion code, having a change pattern in extremely short times. Accordingly, a communication channel utilizing the CDMA technique has high precision timing information owing to a diffusion code. The CDMA mobile communication system is recently advancing in commercialization, and considered to become the core of International Mobile Telecommunications. Thus, those techniques are very useful as a method of realizing acquisition of location information of a mobile station.
However, the conventional technique described in the above-described former patent application document 7-181242, i.e., the technique of using transmission channels from a plurality of base stations whose differences in transmission timing are known in advance, has a problem in that its application to a general mobile communication system is difficult.
The reason for it is as follows. Namely, in order that the differences in channel transmission timing among the base stations can be known in advance, it is necessary to satisfy the condition that the mobile communication system is one in which base stations are precisely synchronized with one another and transmit in transmission timing controlled by a switch or the like. However, as a mobile communication system satisfying such a condition, there is only known IS-95, a CDMA mobile communication system standardized in U.S.A., and a general cellular system or the like does not satisfy this condition.
Further, the conventional technique described in the latter patent application document 10-322752, in which a distance between a mobile station and a base station is obtained by measuring a channel propagation delay of transmission/return-reception, has a problem in that it is difficult to apply it to a case in which it is desirable that the latest possible location information can be obtained at all times such as a guide information service according to a current position. This is because, in order to obtain location information of a mobile station, this method must carry out transmission and return reception every time location information is obtained, and, when location information of a mobile station is updated frequently, the processing load of the system as a whole and use of radio resources become large.
Thus, as described above, the method of the conventional technique is limited in its conditions of the applicable mobile communication system, or its processing load and use of a radio resources become large in a case of frequent use.
An object of the present invention is to provide a mobile communication system that solves the above-described problems of the conventional techniques and can obtain a location of a mobile station without requiring a special condition such as synchronization of a plurality of base stations.
Further, another object of the present invention is to provide a mobile station that can calculate its location and a base station that informs the mobile station about a distance between that base station and the mobile station, without requiring a special condition such as synchronization of a plurality of base stations.
Further, another object of the present invention is to provide a mobile communication system in which location information of a mobile station can be frequently obtained, a mobile station that can calculate its location, and a base station that can inform the mobile station about a distance between that base station and the mobile station, while suppressing the processing load and use of radio resources.
According to the present invention, those objects are achieved by providing a following mobile communication system. Namely, the present invention provides a mobile communication system comprising mobile stations, base stations are switches, wherein:
each of the mobile and base stations has means for retaining and updating a cycle that is common to said mobile and base stations although a phase of said cycle is optional for each station, as a reference cycle;
each of the base stations has means for continuing to transmit a paging channel that has a periodic structure of a same cycle as the reference cycle, designating a phase difference from a phase of the reference cycle of the base station, and including information indicating a transmission source in that paging channel;
each of the mobile stations comprises:
means for receiving the paging channel from the base station, for obtaining a phase difference between a phase of the reference cycle of the mobile station itself and a phase of a cycle of the paging channel at a time of the receiving, as a reception timing of the received paging channel, and for obtaining information that is included in the paging channel and indicates a transmission source of the paging channel;
means for obtaining information on a paging channel transmission timing, which is a phase difference between the phase of the reference cycle of the mobile station itself and a phase of the cycle of the paging channel of the base station at a time of transmission, and for recording the obtained information together with the information indicating the transmission source;
means for obtaining information on a paging channel propagation delay, from the information on the reception timing and transmission timing of the paging channel, when a paging channel for which information on a transmission timing is retained in advance, or when information on the transmission timing of the received paging channel is newly obtained; and
means for obtaining information on a distance between the mobile station itself and the base station, based on the information on the propagation delay of the paging channel and the information indicating the transmission source of the paging channel.
Measurement results obtained from processing required for ordinary communication control for the mobile communication system, such as measurement of a paging channel reception timing and measurement of a propagation delay between a mobile station and a base station, may be diverted to the above-described procedures.
Accordingly, the present invention can obtain location information of a mobile station, with relatively lower processing loads. Further, variations of the above-mentioned means may be used in accordance with setting status of a communication channel from moment to moment. By this, the necessity of dedicated radio resources can be decreased.